The formation of flat or "super flat" concrete floors is the goal of every large construction project. In the construction industry, it is well known that a flat floor will produce a flatness measurement in terms of F(F) numbers in the range of 35-50. Super flat floors, however, will produce F(F) numbers in the range of 50-100. For a detailed description of floor flatness measurements using the industry-standard F-number system, see "Standard Test Method for Determining Floor Flatness and Levelness Using the F-Number System," American Society for Testing and Materials (ASTM) standard designation E 1155.
To achieve flat or super flat concrete floors, the construction industry has for years followed one of two basic methodologies. The first of these will now be explained with the aid of FIG. 1. In FIG. 1, concrete is poured between forms represented by lines 10 and 12 to form a strip 14 of concrete. For reasons that will become apparent below, strip 14 can be up to 50 feet in width but more typically is only 15-30 feet in width and can be as long as the building being constructed (e.g., several hundred feet). Once strip 14 is "struck off" between forms 10 and 12 and has hardened such that only the top 1/8 to 3/16 of an inch remains plastic, the finishing of strip 14 to flat or super flat status is begun.
A riding trowel 16 is positioned on strip 14 and driven thereon to stir up the still-plastic top layer of strip 14. Riding trowel 16 is well known in the art and employs rotating finishing blades or pans in contact with strip 14 to support and propel riding trowel 16 on strip 14. The type of finishing blade(s) chosen is based on the degree of hardness of the concrete with the blade(s) being changed as the concrete gets harder. After each pass of riding trowel 16, it is necessary to cut down high areas of concrete and fill low areas with the excess concrete scraped from the high areas. To do this, workers 20 are positioned on either side of strip 14 and are each equipped with a straight edge tool 22 having a handle 24. One such straight edge tool/handle combination is disclosed by Naser et al. in U.S. Pat. No. 4,702,641. Workers 20 manipulate their straight edge tool 22 (using handle 24) back and forth on strip 14 to cut down high areas of concrete and fill the low areas with the scraped concrete from the high areas.
In recent years, a flat disk or pan has been mounted on riding trowel 16 in place of the blades to form a pan machine as it is known. The pan machine accomplishes the function of several types of blades. This has reduced the number of man-hours required for finishing as the pan machine need not even be used until the later phases of hardening. However, this also means that the window of time for proper cutting of high areas and filling of low areas is reduced thereby making the timing of all finishing operations critical.
Each pass of riding trowel 16 (equipped with either blades or pans) also forms one or more windrows, represented by swirled lines 18, in the wake of riding trowel 16. Windrows 18 are small continuous piles of loosened concrete material that are spun out most prominently to the perimeter of the finishing blades (not shown), or between two non-overlapping pans (not shown), mounted and rotating on riding trowel 16. Since windrows 18 are essentially bumps that disrupt the flatness of strip 14 and since the concrete is in its "almost hard" phase, windrows 18 must be cut and smoothed quickly after their formation. To do this, workers 20 manipulate their straight edge tool 22 (using handle 24) back and forth on strip 14 to smooth out windrows 18. To keep up with riding trowel 16, several workers 20 are typically required on either side of strip 14.
This approach to concrete floor finishing has several disadvantages. Super flat floors can only be achieved if workers 20 stay off strip 14. Therefore, the cut/fill operation and the cutting/smoothing of windrows 18 must be accomplished from either side of strip 14 since effective manipulation of straight edge tool 22 is limited to distances of 15-20 feet. In addition, because windrows 18 are being crossed by straight edge tool 22 at angles between 0.degree. and 45.degree., smaller windrows 19 are continuously created at the outboard ends of straight edge tool 22. Depending on the level of flatness that must be achieved, keeping up with riding trowel 16 can also be a problem as workers 20 may have to work one area of strip 14 more than another. Further, if strip 14 is exposed to a hot sunny environment (as is often the case in summertime floor finishing), windrows 18 can begin to set-up before being cut thereby making the worker's job more difficult or impossible in the worst case scenario.
As mentioned above, the flatness of strip 14 is very important. For this reason, strip 14 is typically only 15-30 feet in width because larger width strips become difficult or impossible to smooth at their centers as workers either cannot reach the center or cannot see what they are doing at the center. Thus, wider width strips (i.e., 30-50 feet) are generally finished at their centers using only riding trowel 16 which limits their flatness to an F(F) number of 30-40 at best. Further, even if the narrower width strips are finished to satisfactory F(F) numbers, adjacent strips necessarily have joints therebetween. These joints can never be finished to the higher F(F) numbers of the strips themselves and therefore will always present potential problems.
Finally, the narrow-width, labor-intensive concrete finishing described above is limited in the amount of concrete that can be finished in a day. In general, an industry-accepted standard is that a crew can finish 9000 square feet per day. However, for large warehouses having 250,000 square feet or more of floor, strip pouring and finishing is not an economical option.
To accommodate large floor construction, the concrete industry in the late 1980's developed a second (now prior art) concrete floor construction that relies on laser screed placement of concrete. In laser screed placement, widths of concrete can exceed 100 feet and recently a pour of 250,000 square feet of concrete was accomplished in a single day. This avoids numerous joints between strips and results in an average cost savings of $1 per square foot. However, these wide-width pours require workers equipped with straight-edge tools to walk on the concrete floor in order to perform the cut/fill operation and the cutting/smoothing of windrows left by the riding trowel. Unfortunately, the damage caused by worker footprints almost offsets the good being achieved by the straight edge tool. Therefore, most contractors rely only on the riding trowel equipped with pans (i.e., a pan machine) to finish a concrete floor that is poured in widths of approximately 50 feet or more. The industry-accepted drawback to finishing only with a riding trowel is a floor flatness that can only achieve an F(F) number in the range of 40-50.